When comparing Opera vs Mozilla Firefox, the Slant community recommends Mozilla Firefox for most people. In the question“What are the best desktop web browsers?”Mozilla Firefox is ranked 2nd while Opera is ranked 8th. The most important reason people chose Mozilla Firefox is:

Pro

VPN onboard

Pro

Strong HTML5 feature support

Pro

Opera Turbo

Opera has a functionality called Turbo which increases browsers speed and reduces data consumption considerably by compressing the data that's found in a web page.

Pro

Battery saving mode

Pro

You can install extensions on the sidebar

Unlike other browsers, you can install add-ons such as tabs, bookmarks, history, sessions, notes, and extension manager in the sidebar. So you have the flexibility to use it for your purpose.

Pro

Opera Sync

Opera Sync allows you to synchronize your bookmarks, passwords, and more from any computer, phone, or tablet.

Pro

Customizable

Opera is full of user controlled settings. Everything can be imported and exported even RSS feeds. It can be skinned in a variety of different themes or it can be extended through the use of a large number of plugins available.

Pro

Speed Dial

Opera's Speed Dial (new tab page) has the ability to hold as many websites as you want, and you can customize it almost however you want. You can also organize the websites on the Speed Dial into folders.

Pro

Portable

You can save the Opera's setup files on a USB hard drive and run its portable version anywhere.

Pro

Uses less resources

Pro

Tagging bookmarks

Firefox is one of the few browsers that you can tag your bookmarks. You can view a list of tags and can search your bookmarks in the address bar with tags.

Pro

Automatically updated

Firefox is automatically updated on the platforms where it makes sense.

Pro

Ethical and pragmatic company mission

The Mozilla Manifesto outlines the company's mission and principles. Paraphrasing, they want the Internet to be a free and open resource, and to enable individuals to get the best use of that resource. They do this by creating open source software to which anyone may contribute, so long as such contributions fit with their principles (both ethical and technical).

Pro

Fast bookmark management

In order to add an open page to the bookmark bar, the tab can be dragged down and is added immediately.

Pro

Lower memory fingerprint than competitors

Firefox used to be a trailer in memory usage, but as of 2017 it's less hungry for memory than competitors like Edge, Chrome, Safari and Opera.

Pro

Text-to-speech (with adjustable speed) without add-ons

Firefox Reader Mode includes Narrate, a feature that adds text-to-speech functionality to the browser.

Pro

Android version allows installation of addons

Unlike other browsers, Mozilla uses almost the same codebase as with the desktop version so extensions work as-is without code modifications - something other browsers cannot do due to their breaking and sometimes unrelated code branches to the mainline desktop branch.

Pro

Easy screenshots without extensions

Within the browser itself, you can easily take screenshots and save them to your computer.

Pro

Screenshots tool

Powerful screenshots tool built right into the browser.

Pro

Dark theme

Pro

Integration with Pocket

Firefox comes with built-in Pocket integration that can allow users to quickly save the article for a read it later function to easily find any articles saved in Pocket from various sources and devices.

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Cons

Con

Owned by a Chinese consortium

Who certainly need to conform with Chinese government requirements to let them infiltrate everything you do online.

Con

WebKit/Blink is not customisable or lightweight

Opera is built on WebKit/Blink, which means it shares cons of Chromium: high RAM usage (Not lightweight), lack of customizability.

Con

Cannot select tabs

Unlike in Chrome and Vivaldi, one cannot select multiple tabs to move to another/new window.

Con

Extension icons not directly moveable

Con

Uneditable default search providers

It can not delete or replace already built-in search services such as Yahoo, Google, DuckDuckGo, and Wikipedia.

Con

Proprietary

While Opera is currently available gratis (without monetary charge), it is currently not libre (meaning that it does not allow users to view the source code used to create, to modify that code, or to redistribute modifications) and is therefor neither free nor open-source software.

Con

Fewer extensions

Con

Buggy

Some buggy or confusing behavior with the bookmarks bar and moving tabs, and entire windows turning black after a while.

Con

Basically a Chrome clone

The latest versions of Opera are basically Chrome clones, so they really don't add anything new.

Con

No address bar in fullscreen mode

Unlike in Vivaldi.

Con

Same security-holes as Chrome

Opera uses the same browser engine as Chrome, meaning it has the same security-holes as Chrome. Chrome is a big target for hackers (being the most popular browser in the world), and a webpage that will hack Chrome will also hack Opera.

Con

No reader view

Con

No menu bar

Con

GTK Themes styles the HTML forms

If you're in Linux and you use a dark GTK theme that uses white text and come to a webpage that forces black text on html-forms buttons you will get black buttons with unreadable black text.

Con

When you search in a website (Ctrl + F) there are no marks appearing in the right scrollbar

All Chromium based browsers have this feature.

Con

Installs Addons with updates

Mozilla is installing/integrating addons with every update like the Mr. Robot promotion - it also has integrated Pocket that spams you every time you open the browser or a new tab with partners of Pocket.

Con

Uses GTK on Linux/BSD

This makes the integration on non-GTK Desktop Environments very hard.

Con

Some built-in advertising

with their new "pocket" feature, they offer advertisements built-in.

Con

Lack of keyboard shortcuts customization

Keyboard shortcuts can not be changed in a user-friendly way.

It is also difficult to manipulate addons with hotkeys.

Con

No status bar

There's no status bar. Granted, there is a status tool-tip. But what if you want an actual toolbar, you're out of luck.

Con

Google obsession

Lately, Firefox has been partnering with Google, leading to some privacy concerns.

Con

Poor performance on SSD even without extensions

On a fast SSD such as Corsair Force LE series this browser still performs as if it were installed on a regular hard drive disk, meaning it takes long time to start, even without any extensions. On that matter Google Chrome is light years ahead of Firefox. This applies only for Firefox versions below 60!

Con

Can't scroll tabs with mouse wheel

It's pretty basic but you can't switch between tabs back and forth using your mouse wheel. Any add-ons supporting this have been deprecated since FF57.

Con

It's a memory hog even though Mozilla claims it is not

Mozilla claims it's using 30% less RAM than Chrome but in real life tests it uses much more.

Con

Devtools for WebExtensions are buggy

Con

Sometimes buggy

Con

Doesn't care for its original guidelines/goals

Mozilla originally aimed to be the "good guys" with user choice and privacy in mind. Their current leadership cannot be trusted to hold those goals in high regard:1) Added Pocket - a privacy data sensitive plugin, made it mandatory2) Tried to sneak in advertisement as "drive-by hack", backpedaled unconvincingly once users complained: https://www.cnet.com/news/mozilla-backpedals-after-mr-robot-firefox-misstep/3) Tried to randomly inject a small percentage of Firefox downloads in Germany with a data collecting plugin (Cliqz) that tech-savy Germans consider adware (no opt-out question asked) : https://bugzilla.mozilla.org/show_bug.cgi?id=1406647

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